Google is reportedly negotiating to purchase the state-owned James R. Thompson Center in downtown Chicago. If true, such a deal could spark a turnaround for the struggling LaSalle Street corridor, where many longtime businesses have recently departed, leaving behind huge vacancies. The state talked for years about selling the 1.2-million-square-foot office center and seemed to reach that goal when Chicago developer Michael Reschke agreed to buy it in March for $70 million. Reschke said he planned to spend nearly $300 million to renovate the Helmut Jahn-designed center, best known for its futur…